Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes such as surface-emitting lasers (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III nitride materials. Typically, III nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
III-nitride devices may be combined with wavelength converting materials such as phosphors, as is known in the art, to form white light or light of other colors.
FIG. 1 illustrates a backlight described in more detail in U.S. Pat. No. 7,052,152. Column 5 line 33 through column 6 line 7 of U.S. Pat. No. 7,052,152 describes the device in FIG. 1 as a backlight configuration when only blue, UV, or near-UV LEDs are used, and where the color-converting phosphor layer 139 is on the cover plate 140. The cover plate 140 may or may not be a diffuser, depending on the amount of diffusion performed by the phosphor. The phosphor layer 139 is a uniform layer, consisting of one or more different type of phosphors. Preferably, a green and a red phosphor are used, but a yellow (YAG) phosphor could be used as well. This layer 139 can, for example, be applied by spray painting, screen-printing, or electrophoretic deposition, or might be a film with uniform density of particles or a luminescent dye distributed throughout the film. This configuration is attractive because the phosphor is not on top of the LED die, and light emitted from the phosphor to the rear of the backlight 126 has a larger recycling efficiency than into the LED chips, due to the high reflectivity of the films used in the backlight 126. And in addition to the recycling efficiency, the phosphor can be operated at a lower temperature and does not have chemical compatibility issues with the LED die, improving the efficiency and lifetime considerably. An LCD panel 114 is disposed over backlight 126.
In another embodiment, one type of phosphor is applied to the cover plate 140, preferably the green or amber phosphor, while another phosphor, preferably the red phosphor, is applied to the rear panel 148 of the backlight configuration. The rear panel acts as a diffuser. This phosphor is not applied as a uniform coating, but is applied as a dot pattern. The combination of blue light from the LEDs and the red and green light from the phosphor layers produces a substantially white backlight for the LCD panel 114. By separating the phosphor in such a configuration, higher conversion efficiency is achieved, while by optimizing the size and spacing of the phosphor dots the required color balance and gamut can be achieved.